Front-end signal compensation

ABSTRACT

A touch surface device having improved sensitivity and dynamic range is disclosed. In one embodiment, the touch surface device includes a touch-sensitive panel having at least one sense node for providing an output signal indicative of a touch or no-touch condition on the panel; a compensation circuit, coupled to the at least one sense node, for generating a compensation signal that when summed with the output signal removes an undesired portion of the output signal so as to generated a compensated output signal; and an amplifier having an inverting input coupled to the output of the compensation circuit and a non-inverting input coupled to a known reference voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/093,638, filed Apr. 7, 2016, and published on Aug. 4, 2016 as U.S.Publication No. 2016/0224185; which is a continuation of U.S. patentapplication Ser. No. 14/042,462, filed Sep. 30, 2013, and issued on Apr.26, 2016 as U.S. Pat. No. 9,323,405; which is a continuation of U.S.patent application Ser. No. 13/284,732, filed Oct. 28, 2011, and issuedon Oct. 8, 2013 as U.S. Pat. No. 8,553,004; which is a continuation ofU.S. patent application Ser. No. 11/650,043, filed Jan. 3, 2007, andissued on Nov. 1, 2011 as U.S. Pat. No. 8,049,732; the entire contentsof which are incorporated herein by reference in their entirety for allpurposes.

FIELD OF THE INVENTION

This relates generally to electronic devices (e.g., a touch screen)capable of generating a dynamic output signal, and more particularly, toa method and system of compensating for undesired portions (e.g., astatic portion) of the output signal.

BACKGROUND OF THE INVENTION

One example of an electronic device that generates dynamic outputsignals is a user input device for performing operations in a computersystem. Such input devices generate output signals based on useroperation of the device or user data or commands entered into thedevice. The operations generally correspond to moving a cursor and/ormaking selections on a display screen. By way of example, the inputdevices may include buttons or keys, mice, trackballs, touch pads, joysticks, touch screens and the like. Touch pads and touch screens(collectively “touch surfaces”) are becoming increasingly popularbecause of their ease and versatility of operation as well as to theirdeclining price. Touch surfaces allow a user to make selections and movea cursor by simply touching the surface, which may be a pad or thedisplay screen, with a finger, stylus, or the like. In general, thetouch surface recognizes the touch and position of the touch and thecomputer system interprets the touch and thereafter performs an actionbased on the touch.

Touch pads are well-known and ubiquitous today in laptop computers, forexample, as a means for moving a cursor on a display screen. Such touchpads typically include a touch-sensitive opaque panel which senses whenan object (e.g., finger) is touching portions of the panel surface.Touch screens are also well known in the art. Various types of touchscreens are described in applicant's co-pending patent application Ser.No. 10/840,862, entitled “Multipoint Touchscreen,” filed May 6, 2004,which is hereby incorporated by reference in its entirety. As notedtherein, touch screens typically include a touch-sensitive panel, acontroller and a software driver. The touch-sensitive panel is generallya clear panel with a touch sensitive surface. The touch-sensitive panelis positioned in front of a display screen so that the touch sensitivesurface covers the viewable area of the display screen. Thetouch-sensitive panel registers touch events and sends these signals tothe controller. The controller processes these signals and sends thedata to the computer system. The software driver translates the touchevents into computer events. There are several types of touch screentechnologies including resistive, capacitive, infrared, surface acousticwave, electromagnetic, near field imaging, etc. Each of these deviceshas advantages and disadvantages that are taken into account whendesigning or configuring a touch screen.

In conventional touch surface devices, and other types of input devices,there is typically an operational amplifier that amplifies the outputsignal of the device. The output signal is a dynamic signal in that itchanges between two or more states (e.g., a “touch” or “no touch”condition). In conventional devices, the amplifier may be followed by anoutput signal compensation circuit that provides a compensation signalto offset an undesired portion (e.g., static portion) of the outputsignal. The problem with this configuration is that the amplifieramplifies both the dynamic signal of interest as well as the undesiredstatic or offset portion.

Additionally, by compensating the output signal after it has beenamplified, conventional compensation methods provide poor utilization ofthe output dynamic range of the amplifier, which results in poorsensitivity in detecting dynamic changes in the output signal.

Furthermore, in devices wherein the output signal is a charge waveform(e.g., an output signal from a capacitive touch surface), a relativelylarge feedback capacitor is typically connected between the output ofthe amplifier and the inverting input of the amplifier in order toaccommodate relatively large charge amplitudes at the inverting input ofthe amplifier. The charge amplitudes should be sufficiently large toprovide a sufficiently high signal-to-noise (S/N) ratio. The largefeedback capacitors, however, consume a significant amount of integratedcircuit (IC) chip “real estate” and hence, add significant costs andsize requirements to the IC chips.

SUMMARY OF THE INVENTION

The invention addresses the above and other needs by providing a newmethod and system for compensating for undesired portions (i.e., “offsetportions”) of an output signal. In various embodiments, the invention isutilized in connection with a touch surface device, wherein offsetcompensation is provided to the output signals of the touch surfacedevice before the output signal is provided to an input of an amplifier.Thus, the amplifier amplifies only a desired (e.g., dynamic) portion ofthe output signal. When the output signal is compensated in thisfashion, changes in magnitude of the output signals due to a touch ofthe touch surface device, for example, reflect a much larger portion ofthe dynamic range of the amplifier, thereby providing more sensitivityand dynamic range to the touch surface device.

In one embodiment, the output signal of a touch surface device is summedwith a compensation signal prior to being provided to an inverting inputof an amplifier. The compensation signal has a desired amplitude,waveform, frequency and phase to provide a desired compensation to theoutput signal. In one embodiment, the compensation signal is generatedby a compensation circuit that includes a look-up table, a digital toanalog voltage converter (VDAC) and a compensation capacitor C_(COMP)for converting the output of the VDAC into a charge waveform that isused to compensate a charge waveform output of the touch surface device.The look-up table stores digital codes that are provided to the VDAC togenerate the desired compensation signal.

In another embodiment, a charge compensation circuit includes a look-uptable and a digital-to-analog current converter (IDAC). The look-uptable stores digital codes that are provided to the IDAC to generate adesired current waveform that when viewed in the charge domaincorresponds to a desired charge waveform to compensate a charge waveformoutput signal.

In a further embodiment, a compensation signal is generated by one ormore capacitive nodes on a touch surface device that are insensitive totouch. A compensation drive signal, provided to one or moretouch-insensitive nodes, is substantially 180 degrees out of phase withthe drive signal provided to the touch-sensitive nodes of the touchsurface device. The touch-insensitive nodes provide a compensationsignal that is substantially 180 degrees out of phase with respect to anoutput signal generated by a touch sensitive node such that when summedtogether, a desired portion of the output signal is removed.Additionally, because the compensation signal is being generated by thetouch surface device, variations in the output signal from atouch-sensitive node due to variations in processing or externalconditions (e.g., temperature, dielectric thickness, etc.) are alsoexhibited by the compensation signal. Thus, the behavior and/orvariations in the compensation signal “track” the behavior and/orvariations in the output signals generated by the touch-sensitiveportions of the touch surface device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of a touch surface device capableutilizing an improved output signal compensation circuit and method, inaccordance with one embodiment of the invention.

FIG. 2 is a block diagram of a computing device or system incorporatinga touch surface device, in accordance with one embodiment of theinvention.

FIGS. 3A and 3B illustrate two possible arrangements of drive and senseelectrodes in a touch screen, in accordance with various embodiments ofthe invention.

FIG. 4 illustrates a top view of transparent multipoint touch screen, inaccordance with one embodiment of the present invention.

FIG. 5 is a partial front elevation view, in cross section of a displayarrangement, in accordance with one embodiment of the present invention.

FIG. 6 is a simplified diagram of a mutual capacitance circuit, inaccordance with one embodiment of the present invention.

FIG. 7 is a diagram of a charge amplifier, in accordance with oneembodiment of the present invention.

FIG. 8 is a block diagram of a touch surface device and controllersystem, in accordance with one embodiment of the present invention.

FIGS. 9A and 9B illustrate perspective side views of an exemplarycapacitive sensing nodes (a.k.a., pixels) in “no touch” and “touch”states, respectively, in accordance with one embodiment of the presentinvention.

FIG. 10A illustrates an exemplary drive signal waveform applied to aselected drive (e.g., row) electrode of a touch surface panel, inaccordance with one embodiment of the present invention.

FIG. 10B illustrates exemplary charge output waveforms (“touch” and “notouch”) generated by a sense (e.g., column) electrode of a touch surfacepanel, in accordance with one embodiment of the present invention.

FIG. 11 illustrates an exemplary analog sensing circuit or channel withfront-end compensation, in accordance with one embodiment of the presentinvention.

FIG. 12 illustrates exemplary compensated signal waveforms representinga “no touch” and “max touch” state, respectively, in accordance with oneembodiment of the present invention.

FIG. 13A illustrates an exemplary compensation signal generator circuit,in accordance with one embodiment of the present invention.

FIG. 13B illustrates another exemplary compensation signal generatorcircuit, in accordance with another embodiment of the present invention.

FIG. 14 illustrates a touch surface device and its drive circuitry,wherein portions of the touch surface device are utilized to generate acompensation signal, in accordance with one embodiment of the presentinvention.

FIG. 15 illustrates a top view of an exemplary touch surface panelwherein a top row of the touch surface panel is utilized to generate acompensation signal, in accordance with one embodiment of the invention.

FIGS. 16A and 16B illustrate a touch-sensitive capacitive sensing nodeand a touch-insensitive node that is utilized to generate at least aportion of a compensation signal, in accordance one embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description of preferred embodiments, reference is madeto the accompanying drawings which form a part hereof, and in which itis shown by way of illustration specific embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention. Furthermore, althoughembodiments of the present invention are described herein in terms ofdevices and applications compatible with computer systems and devicesmanufactured by Apple Computer, Inc. of Cupertino, Calif., suchembodiments are illustrative only and should not be considered limitingin any respect.

FIG. 1 is a perspective view of a touch screen display arrangement 30,which includes a display 34 and a transparent touch screen 36 positionedin front of display 34. Display 34 may be configured to display agraphical user interface (GUI) including perhaps a pointer or cursor aswell as other information to the user. Transparent touch screen 36 is aninput device that is sensitive to a user's touch, allowing a user tointeract with the graphical user interface on display 34. In general,touch screen 36 recognizes touch events on surface 38 of touch screen 36and thereafter outputs this information to a host device. The hostdevice may, for example, correspond to a computer such as a desktop,laptop, handheld or tablet computer. The host device interprets thetouch event and thereafter performs an action based on the touch event.

In one embodiment, touch screen 36 is configured to recognize multipletouch events that occur simultaneously at different locations on touchsensitive surface 38. That is, touch screen 36 allows for multiplecontact points T1-T4 to be tracked simultaneously. Touch screen 36generates separate tracking signals S1-S4 for each touch point T1-T4that occurs on the surface of touch screen 36 at the same time. In oneembodiment, the number of recognizable touches may be about fifteenwhich allows for a user's ten fingers and two palms to be tracked alongwith three other contacts. The multiple touch events can be usedseparately or together to perform singular or multiple actions in thehost device. Numerous examples of multiple touch events used to controla host device are disclosed in U.S. Pat. Nos. 6,323,846; 6,888,536;6,677,932; 6,570,557, and co-pending U.S. patent application Ser. Nos.11/015,434; 10/903,964; 11/048,264; 11/038,590; 11/228,758; 11/228,700;11/228,737; 11/367,749, each of which is hereby incorporated byreference in its entirety.

FIG. 2 is a block diagram of a computer system 50, employing amulti-touch touch screen. Computer system 50 may be, for example, apersonal computer system such as a desktop, laptop, tablet, or handheldcomputer. The computer system could also be a public computer systemsuch as an information kiosk, automated teller machine (ATM), point ofsale machine (POS), industrial machine, gaming machine, arcade machine,vending machine, airline e-ticket terminal, restaurant reservationterminal, customer service station, library terminal, learning device,mobile telephone, audio/video player, etc.

Computer system 50 includes a processor 56 configured to executeinstructions and to carry out operations associated with the computersystem 50. Computer code and data required by processor 56 are generallystored in storage block 58, which is operatively coupled to processor56. Storage block 58 may include read-only memory (ROM) 60, randomaccess memory (RAM) 62, hard disk drive 64, and/or removable storagemedia such as CD-ROM, PC-card, floppy disks, and magnetic tapes. Any ofthese storage devices may also be accessed over a network. Computersystem 50 also includes a display device 68 that is operatively coupledto the processor 56. Display device 68 may be any of a variety ofdisplay types including liquid crystal displays (e.g., active matrix,passive matrix, etc.), cathode ray tubes (CRT), plasma displays, etc.Computer system 50 also includes touch screen 70, which is operativelycoupled to the processor 56 by I/O controller 66 and touch screencontroller 76. (The I/O controller 66 may be integrated with theprocessor 56, or it may be a separate component.) In any case, touchscreen 70 is a transparent panel that is positioned in front of thedisplay device 68, and may be integrated with the display device 68 orit may be a separate component. Touch screen 70 is configured to receiveinput from a user's touch and to send this information to the processor56. In most cases, touch screen 70 recognizes touches and the positionand magnitude of touches on its surface.

The host processor 561 receives outputs from the touch screen controller76 and performs actions based on the outputs. Such actions may include,but are not limited to, moving an object such as a cursor or pointer,scrolling or panning, adjusting control settings, opening a file ordocument, viewing a menu, making a selection, executing instructions,operating a peripheral device connected to the host device, answering atelephone call, placing a telephone call, terminating a telephone call,changing the volume or audio settings, storing information related totelephone communications such as addresses, frequently dialed numbers,received calls, missed calls, logging onto a computer or a computernetwork, permitting authorized individuals access to restricted areas ofthe computer or computer network, loading a user profile associated witha user's preferred arrangement of the computer desktop, permittingaccess to web content, launching a particular program, encrypting ordecoding a message, and/or the like. The host processor 76 may alsoperform additional functions that may not be related to multi-touch (MT)panel processing, and may be coupled to program storage 58 and thedisplay device 68 such as an LCD display for providing a user interface(UI) to a user of the device.

In one embodiment, the touch screen panel 70 can be implemented as amutual capacitance device constructed as described below with referenceto FIGS. 3A and 3B. In this embodiment, the touch screen panel 70 iscomprised of a two-layered electrode structure, with driving lines orelectrodes on one layer and sensing lines or electrodes on the other. Ineither case, the layers are separated by a dielectric material (notshown). In the Cartesian arrangement of FIG. 3A, one layer is comprisedof N horizontal, preferably equally spaced row electrodes 81, while theother layer is comprised of M vertical, preferably equally spaced columnelectrodes 82. In a polar arrangement, illustrated in FIG. 3B, thesensing lines may be concentric circles and the driving lines may beradially extending lines (or vice versa). As will be appreciated bythose skilled in the art, other configurations based on a variety ofcoordinate systems are also possible. Additionally, it is understoodthat the invention is not necessarily limited to touch surface devicesutilizing mutual capacitance sensing nodes. The invention may beimplemented within other types of touch surface devices such as “selfcapacitance” devices, for example.

Each intersection 83 represents a pixel and has a characteristic mutualcapacitance, C_(SIG). A grounded object (such as a finger) thatapproaches a pixel 83 from a finite distance shunts the electric fieldbetween the row and column intersection, causing a decrease in themutual capacitance C_(SIG) at that location. In the case of a typicalsensor panel, the typical signal capacitance C_(SIG) is about 1.0picofarads (pF) and the change (ΔC_(SIG)) induced by a finger touching apixel, is about 0.10 pF. These capacitance values are exemplary only andshould not in any way limit the scope of the present invention.

The electrode material may vary depending on the application. In touchscreen applications, the electrode material may be ITO (Indium TinOxide) on a glass substrate. In a touch tablet, which need not betransparent, copper on an FR4 substrate may be used. The number ofsensing points 83 may also be widely varied. In touch screenapplications, the number of sensing points 83 generally depends on thedesired sensitivity as well as the desired transparency of the touchscreen 70. More nodes or sensing points generally increases sensitivity,but reduces transparency (and vice versa).

During operation, each row electrode (i.e., a drive electrode) issequentially charged by driving it with a predetermined voltage waveform(discussed in greater detail below). The charge capacitively couples tothe column electrodes (i.e., sense electrodes) at the intersectionsbetween the drive electrode and the sense electrodes. In alternativeembodiments the column electrodes can be configured as the driveelectrodes and the row electrodes can be configured as the senseelectrodes. The capacitance of each intersection 83 is measured todetermine the positions of multiple objects when they touch the touchsurface. Sensing circuitry monitors the charge transferred and timerequired to detect changes in capacitance that occur at each node. Thepositions where changes occur and the magnitude of those changes areused to identify and quantify the multiple touch events.

FIG. 4 is a top view of a transparent multipoint touch screen 150, inaccordance with one embodiment of the present invention. As shown, thetouch screen 150 includes a two layer grid of spatially separated linesor wires 152. In most cases, the lines 152 on each layer are parallelone another. Furthermore, although in different planes, the lines 152 onthe different layers are configured to intersect or cross in order toproduce capacitive sensing nodes 154 (a.k.a., “pixels”), which eachrepresent different coordinates in the plane of the touch screen 150.The nodes 154 are configured to receive capacitive input from an objecttouching the touch screen 150 in the vicinity of the node 154. When anobject (e.g., a finger tip) is proximate the node 154, the object stealscharge thereby affecting the capacitance at the node 154. It has beenfound that as a finger is pressed more firmly against the touch screensurface 150, the surface area of the finger touching the touch screen150 increases and a greater amount of charge is diverted away from theunderlying sensing node(s) 154.

The lines 152 on different layers serve two different functions. One setof lines 152A drives a current therethrough while the second set oflines 152B senses the capacitance coupling at each of the nodes 154. Inmost cases, the top layer provides the driving lines 152A while thebottom layer provides the sensing lines 152B. The driving lines 152A areconnected to a voltage source (not shown) that separately drives thecurrent through each of the driving lines 152A. That is, the stimulus isonly happening over one line while all the other lines are grounded.They may be driven similarly to a raster scan. Each sensing line 152B isconnected to a capacitive sensing circuit (not shown) that senses acharge and, hence, capacitance level for the sensing line 152B.

When driven, the charge on the driving line 152A capacitively couples tothe intersecting sensing lines 152B through the nodes 154 and thecapacitive sensing circuits sense their corresponding sensing lines 152Bin parallel. Thereafter, the next driving line 152A is driven, and thecharge on the next driving line 152A capacitively couples to theintersecting sensing lines 152B through the nodes 154 and the capacitivesensing circuits sense all of the sensing lines 152B in parallel. Thishappens sequentially until all the lines 152A have been driven. Once allthe lines 152A have been driven, the sequence starts over (continuouslyrepeats). As explained in further detail below, in one embodiment, thecapacitive sensing circuits are fabricated on an application specificintegrated circuit (ASIC), which converts analog capacitive signals todigital data and thereafter transmits the digital data over a serial busto a host controller or microprocessor for processing.

The lines 152 are generally disposed on one or more optical transmissivemembers 156 formed from a clear material such as glass or plastic. Byway of example, the lines 152 may be placed on opposing sides of thesame member 156 or they may be placed on different members 156. Thelines 152 may be placed on the member 156 using any suitable patterningtechnique including for example, deposition, etching, printing and thelike. Furthermore, the lines 152 can be made from any suitabletransparent conductive material. By way of example, the lines may beformed from indium tin oxide (ITO). The driving lines 152A may becoupled to the voltage source through a flex circuit 158A, and thesensing lines 152B may be coupled to the sensing circuits via a flexcircuit 158B. The sensor ICs may be attached to a printed circuit board(PCB).

The distribution of the lines 152 may be widely varied. For example, thelines 152 may be positioned almost anywhere in the plane of the touchscreen 150. The lines 152 may be positioned randomly or in a particularpattern about the touch screen 150. With regards to the later, theposition of the lines 152 may depend on the coordinate system used. Forexample, the lines 152 may be placed in rows and columns for Cartesiancoordinates or concentrically and radially for polar coordinates. Whenusing rows and columns, the rows and columns may be placed at variousangles relative to one another. For example, they may be vertical,horizontal or diagonal.

FIG. 5 is a partial front elevation view, in cross section of anexemplary display arrangement 170. The display arrangement 170 includesan LCD display 172 and a touch screen 174 positioned over the LCDdisplay 172. The touch screen may for example correspond to the touchscreen shown in FIG. 4. The LCD display 172 may correspond to anyconventional LCD display known in the art. Although not shown, the LCDdisplay 172 typically includes various layers including a fluorescentpanel, polarizing filters, a layer of liquid crystal cells, a colorfilter and the like.

The touch screen 174 includes a transparent sensing layer 176 that ispositioned over a first glass member 178. The sensing layer 176 includesa plurality of sensor lines 177 positioned in columns (which extend inand out of the page). The first glass member 178 may be a portion of theLCD display 172 or it may be a portion of the touch screen 174. Forexample, it may be the front glass of the LCD display 172 or it may bethe bottom glass of the touch screen 174. The sensor layer 176 istypically disposed on the glass member 178 using suitable transparentconductive materials and patterning techniques. In some cases, it may bedesirable to coat the sensor layer 176 with material of similarrefractive index to improve the visual appearance, i.e., make it moreuniform.

The touch screen 174 also includes a transparent driving layer 180 thatis positioned over a second glass member 182. The second glass member182 is positioned over the first glass member 178. The sensing layer 176is therefore sandwiched between the first and second glass members 178and 182. The second glass member 182 provides an insulating layerbetween the driving and sensing layers 176 and 180. The driving layer180 includes a plurality of driving lines 181 positioned in rows (extendto the right and left of the page). The driving lines 181 are configuredto intersect or cross the sensing lines 177 positioned in columns inorder to form a plurality of capacitive coupling nodes 182. Like thesensing layer 176, the driving layer 180 is disposed on the glass member182 using suitable materials and patterning techniques. Furthermore, insome cases, it may be necessary to coat the driving layer 180 withmaterial of similar refractive index to improve the visual appearance.Although the sensing layer is typically patterned on the first glassmember, it should be noted that in some cases it may be alternatively oradditionally patterned on the second glass member.

The touch screen 174 also includes a protective cover sheet 190 disposedover the driving layer 180. The driving layer 180 is thereforesandwiched between the second glass member 182 and the protective coversheet 190. The protective cover sheet 190 serves to protect the underlayers and provide a surface for allowing an object to slide thereon.The protective cover sheet 190 also provides an insulating layer betweenthe object and the driving layer 180. The protective cover sheet issuitably thin to allow for sufficient coupling. The protective coversheet 190 may be formed from any suitable clear material such as glassand plastic. In addition, the protective cover sheet 190 may be treatedwith coatings to reduce friction or sticking when touching and reduceglare when viewing the underlying LCD display 172. By way of example, alow friction/anti reflective coating may be applied over the cover sheet190. Although the line layer is typically patterned on a glass member,it should be noted that in some cases it may be alternatively oradditionally patterned on the protective cover sheet.

The touch screen 174 also includes various bonding layers 192. Thebonding layers 192 bond the glass members 178 and 182 as well as theprotective cover sheet 190 together to form the laminated structure andto provide rigidity and stiffness to the laminated structure. Inessence, the bonding layers 192 help to produce a monolithic sheet thatis stronger than each of the individual layers taken alone. In mostcases, the first and second glass members 178 and 182 as well as thesecond glass member and the protective sheet 182 and 190 are laminatedtogether using a bonding agent such as glue. The compliant nature of theglue may be used to absorb geometric variations so as to form a singularcomposite structure with an overall geometry that is desirable. In somecases, the bonding agent includes an index matching material to improvethe visual appearance of the touch screen 170.

FIG. 6 is a simplified diagram of a mutual capacitance circuit 220, inaccordance with one embodiment of the present invention. The mutualcapacitance circuit 220 includes a driving line 222 and a sensing line224 that are spatially separated by a capacitive coupling node 226. Thedriving line 222 is electrically coupled to a voltage source 228, andthe sensing line 224 is electrically coupled to a capacitive sensingcircuit 230. The driving line 222 is configured to carry a current tothe capacitive coupling node 226, and the sensing line 224 is configuredto carry a current to the capacitive sensing circuit 230. When no objectis present, the capacitive coupling at the node 226 stays fairlyconstant. When an object 232 such as a finger is placed proximate thenode 226, the capacitive coupling through the node 226 changes. Theobject 232 effectively shunts some of the electromagnetic field away sothat the charge formed across the node 226 decreases. The change incapacitive coupling changes the current that is carried by the sensinglines 224. The capacitive sensing circuit 230 notes the current changeand the position of the node 226 where the current change occurred andreports this information in a raw or in some processed form to a hostcontroller or microprocessor. Such sensing occurs for each node at arapid scan rate so that from the perspective of a user it appears thatall nodes are sensed simultaneously.

In one embodiment, the capacitive sensing circuit 230 includes an inputfilter 236 for eliminating parasitic or stray capacitance 237, which mayfor example be created by the large surface area of the row and columnlines relative to the other lines and the system enclosure at groundpotential. Generally speaking, the filter rejects stray capacitanceeffects so that a clean representation of the charge transferred acrossthe node 226 is outputted. That is, the filter 236 produces an outputthat is not dependent on the parasitic or stray capacitance, but ratheron the capacitance at the node 226. As a result, a more accurate outputis produced.

FIG. 7 is a diagram of a charge amplifier 240 that may be used as thefilter 236, in accordance with one embodiment of the present invention.As shown, the amplifier includes a non inverting input that is held at aconstant voltage (e.g., a reference voltage or ground), and an invertinginput that is coupled to the node 226. The output of the amplifier 240is coupled back to the inverting input through a feedback capacitor(C_(FB)) 242. As is known in the art, in this configuration, theamplifier 240 and C_(FB) 242 eliminate stray capacitance that mayotherwise effect the measurement of capacitance or change in capacitance(ΔC_(SIG)) at the capacitive sensing node 226. Because of thecharacteristics of the amplifier 240, any charge that appears acrossC_(STRAY) will be equal to the charge at the output of the amplifierand, therefore, no matter how much stray capacitance C_(STRAY) is addedto the inverting input, the net charge across C_(SIG) will always bezero. In one embodiment, the inverting amplifier 240 in combination withthe C_(FB) 242 perform the following tasks: (1) charge to voltageconversion, (2) charge amplification, (3) rejection of stray capacitancepresent at the column electrode, (4) anti aliasing, and (5) gainequalization at different frequencies. Charge to voltage conversion isperformed by the feedback capacitor C_(FB) in the feedback path of theamplifier 240.

In one embodiment, the functions of driving each row electrode andsensing the charge transfer on each corresponding column electrode areperformed by a multipoint touch screen controller system 300, as shownin FIG. 8. The controller system 100 includes a touch surface device 301(e.g., a touch screen or touch pad), an application specific integratedcircuit (ASIC) chip 305 for receiving output (e.g., touch data) from thetouch surface 301, a microprocessor 307 for receiving and processingdigital data from the ASIC 305, a level shifter or voltage multiplier310 for generating drive signals of a desired amplitude, and a decoder311 for decoding timing signals from the microprocessor and applying thedrive signal to an appropriate row electrode.

In one embodiment, the ASIC 305 receives analog signals (e.g., voltagewaveforms) from each column electrode 82 (FIG. 3a ) of the touch surfacedevice 301 indicating a touch or no-touch condition at a respectivecapacitive sensing node 83 corresponding to an intersection of thecolumn electrode 82 and a selected, driven row electrode 81 of the touchsurface device 301. The ASIC 305 converts the analog signals receivedfrom the node 83 of the touch surface 301 into digital signals which arethen received and processed by the microprocessor 307 in order to sensetouch and/or multi-touch states. In one embodiment, the ASIC 305contains a plurality of inverting amplifier 240 and feedback capacitor242 circuits, similar to that shown in FIG. 7, each coupled torespective column electrodes of the touch surface device 301.

The ASIC 305 further generates all the drive waveforms necessary to scanthe sensor panel and provides those waveforms to the level shifter 310,which amplifies the drive waveforms. In one embodiment, themicroprocessor 307 sends a clock signal 321 to set the timing of theASIC 305, which in turn generates the appropriate timing waveforms 322to create the row stimuli to the touch surface device 301. Decoder 311decodes the timing signals to drive each row of the touch surface 301 insequence. Level shifter 310 converts the timing signals 322 from thesignaling level (e.g., 3.3 V_(p-p)) to the level used to drive the touchsurface device 301 (e.g., 18V_(p-p)).

In one embodiment, it is desirable to drive the panel at multipledifferent frequencies for noise rejection purposes. Noise that exists ata particular drive frequency may not, and likely will not exist at theother frequencies. In one embodiment, each sensor panel row isstimulated with three bursts of twelve square wave cycles (50%duty-cycle, 18V amplitude), while the remaining rows are kept at ground.For better noise rejection, the frequency of each burst is different.Exemplary burst frequencies are 140 kHz, 200 kHz, and 260 Khz. A moredetailed discussion of this “frequency hopping” method is provided in acommonly-owned and concurrently pending patent application entitled“Scan Sequence Generator” (U.S. application Ser. No. 11/650,046), theentirety of which is incorporated by reference herein.

During each burst of pulses, ASIC 305 takes a measurement of the columnelectrodes. This process is repeated for all remaining rows in thesensor panel. After all rows have been scanned in a single scan cycle,the measurement results are used to provide one or more images of thetouch/no-touch state of the touch surface 301, each image taken at adifferent stimulus frequency. The images are stored in a memory (notshown) accessible by the microprocessor 307 and processed to determine ano-touch, touch or multi-touch condition.

FIG. 9A illustrates a side view of an exemplary sensing node (a.k.a.,pixel) 350 in a steady-state (no-touch) condition. The node 350 islocated at an intersection of a row electrode 352 and a column electrode354, separated by a dielectric 356. An electric field illustrated byelectric field lines 358 between the column 354 and row 352 traces orelectrodes create a mutual capacitance, C_(SIG), between the row andcolumn electrodes when a driving signal or stimulus is applied to therow electrode or trace 352.

FIG. 9B is a side view of the exemplary node 350 in a dynamic (touch)condition. A user's finger 360, which has been placed on or near thenode 350, blocks some of the electric field lines 358 between the row352 and column 354 electrodes (those fringing fields that exit thedielectric and pass through the air above the row electrode). Theseelectric field lines are shunted to ground through the capacitanceinherent in the finger, and as a result, the steady state signalcapacitance C_(SIG) is reduced by ΔC_(SIG). Therefore, the signalcapacitance at the node 350 becomes C_(SIG)−ΔC_(SIG), where C_(SIG)represents the static (no touch) component and ΔC_(SIG) represents thedynamic (touch) component. Note that C_(SIG)−ΔC_(SIG) may always benonzero due to the inability of a finger or other object to block allelectric fields, especially those electric fields that remain entirelywithin the dielectric. In addition, it should be understood that as afinger is pushed harder or more completely onto the touch surface, thefinger will tend to flatten and increase in surface area, therebyblocking more of the electric fields. Thus, ΔC_(SIG) may be variable andrepresentative of how completely the finger is pushing down on the panel(i.e., a range from “no-touch” to “full-touch”).

FIG. 10A illustrates an exemplary driving signal waveform 402 that maybe provided to a row electrode 81 (FIG. 3A). In this example, thedriving signal is a square wave signal having a peak-to-peak amplitudeof approximately 20 V_(p-p). Since charge (Q) equals voltage (V)multiplied by capacitance (C), if the mutual capacitance (C_(SIG)) at asensing node 83 is 1.0 pF, for example, the output at the correspondingcolumn electrode 82 will be a square wave having an amplitude of 20pico-coulombs peak-to-peak (pC_(p-p)) when viewed in the charge domain,as shown by solid line 404 in FIG. 10B. When a finger or other objectcomes in close proximity to the node 83, its mutual capacitance valuewill decrease to C_(SIG)−ΔC_(SIG), as discussed above. If, for example,the decreased capacitance C_(SIG)−ΔC_(SIG)=0.9 pF, then the output atthe column electrode 82 will be a square wave having a peak-to-peakamplitude of 18 pC_(p-p), as indicated by the dashed waveform 406 inFIG. 10B (these figures are not necessarily drawn to scale). Thus, thedifference in charge output from a column electrode between a “no-touch”condition and a “touch” condition would be 20−18=2 pC_(p-p) in thisexample.

FIG. 11 illustrates a capacitive sensing circuit 500 with front-endcharge compensation, in accordance with one embodiment of the invention.Depending on whether a touch condition is being sensed at a respectivecapacitive sensing node, as described above, a charge waveform 404 (notouch) or 406 (touch) will be received at a first input of a signalsumming circuit or multiplier 502. The sensing circuit 500 furtherincludes a digital-analog-converter (DAC) 504 which provides acompensation waveform signal 506 to a second input of the multiplier502. The compensation waveform signal 506 is generated to havesubstantially the same frequency as the charge waveform 404 or 406 butsubstantially 180 degrees out of phase with the charge waveform 404 or406 outputted from the touch surface device 301 (FIG. 8). The amplitudeof the compensation signal 506 may be selected to achieve any desiredlevel of offset and/or amplitude range for the resulting compensatedsignal 508 outputted by the multiplier 502. In one embodiment, thepeak-to-peak amplitude of the compensation signal 506 is selected to bethe average of the amplitudes of waveforms 404 and 406. For example, ifthe amplitude of waveform 404 is 20 pC_(p-p) and the amplitude ofwaveform 406 is 18 pC_(p-p) (e.g., at full touch), the amplitude of thecompensation waveform 506 would be selected to be 19 pC_(p-p). Variousadvantages of selecting the amplitude of the compensation waveform 506to be the average of the amplitudes of waveforms 404 and 406 arediscussed in further detail below.

The capacitive sensing circuit 500 further includes a look-up table 510that provides a digital signal to the DAC 504, which the DAC 504converts into the desired compensation waveform 506, having a desiredamplitude, shape, frequency and phase. Various embodiments of the DAC504 are described in further detail below. In one embodiment, thelook-up table 510 is pre-programmed to provide digital codes to the DAC504 to generate predetermined compensation waveforms 506 correspondingto each drive signal frequency. A control signal 511 generated by row orchannel scan logic circuitry (not shown) within the ASIC 305 controlswhat outputs will be provided to the DAC 504 and mixer 512 (describedbelow). In various embodiments, the look-up table 510 may be implementedas one or more look-up tables residing in a memory of the ASIC 305.Thus, in the embodiment illustrated in FIG. 11, a front-end compensationcircuit is provided that includes the summing circuit 502, the DAC 504and the look-up table 510.

The compensated output signal 508 from the summing circuit 502 isprovided to an inverting input of the operational amplifier 240. Sincethe compensated signal 508 is a charge waveform, the feedback capacitor242 converts the charge waveform into a voltage waveform according tothe equation Q=C_(FB)V_(out). or V_(out).=Q/C_(FB), where Q is theamplitude of the compensated waveform 508 and V_(out). is the amplitudeof the resulting voltage waveform 512 at the output of the amplifier240. It will be appreciated that since the peak-to-peak amplitude of thecompensated waveform 508 (e.g., 0-2 pC_(p-p)) is significantly smallerthan the amplitude of the uncompensated waveforms 404 or 406, the valueof C_(FB) may be significantly reduced (e.g., by a factor of 10-20times) while maintaining desired voltage ranges (e.g., CMOS levels) forV_(out) 512 at the output of the amplifier 240. For example, to achievea dynamic range of 1 volt, peak-to-peak (V_(p-p)) at the output of theamplifier 240, if the signal at the inverting input of the amplifier is20 pC_(p-p) (uncompensated), then C_(FB) must be equal to 20 pF. Incontrast, if the maximum amplitude of the signal at the inverting inputof the amplifier 240 is 2 pC_(p-p) (compensated), then C_(FB) must onlyequal 2 pF to provide a dynamic range of 1 V_(p-p). This reduction insize of C_(FB) is a significant advantage in terms of chip cost and“real estate” for the ASIC 305 (FIG. 8), which, in one embodiment, cancontain multiple capacitive sensing circuits 500 at its input stage.Capacitors require a relatively large die area in integrated circuits,which add to their costs and limit the number of devices (e.g.,transistors) that can be integrated onto the IC chip. Therefore, it isadvantageous to decrease the size of capacitors in IC chips whenpossible.

As shown in FIG. 11, the output waveform 511 of the amplifier 240 isprovided to a first input of a mixer 514. Since the waveform 511 is asquare wave, which may create undesirable harmonics, a demodulationwaveform 516, which may be a sine wave digitally generated from thelook-up table 510, is synchronized to the output signal 511 and providedto a second input of the mixer 514. In one embodiment, the mixer 514demodulates the output 511 of the charge amplifier by subtracting thedemodulation waveform 516 from the output signal 511 to provide betternoise rejection. The mixer 514 rejects all frequencies outside itspassband, which may be about +/−30 kHz around the frequency of thedemodulation waveform 516. This noise rejection may be beneficial innoisy environment with many sources of noise, such as 802.11, Bluetooth,etc. In some embodiments, the mixer 514 may be implemented as asynchronous rectifier that outputs a rectified Gaussian sine wave. Theoutput of the mixer is provided to an analog-to-digital converter (ADC)518, which converts the analog signals into corresponding digitalsignals for storage and processing by the microprocessor 307 (FIG. 8).

In one embodiment, the ADC 518 may be a sigma-delta converter, which maysum a number of consecutive digital values and average them to generatea result. However, other types of ADCs (such as a voltage to frequencyconverter with a subsequent counter stage) could be used. The ADCtypically performs two functions: (1) it converts the offset compensatedwaveform from the mixer 514 to a digital value; and (2) it performs lowpass filtering functions, e.g., it averages a rectified signal comingout of the mixer arrangement. The offset compensated, demodulated signallooks like a rectified Gaussian shaped sine wave, whose amplitude is afunction of C_(FB) and C_(SIG). The ADC result returned to themicroprocessor 307 is typically the average of that signal.

It is appreciated that the front-end charge compensation provided by thesumming circuit 502 also significantly improves utilization of thedynamic range of the amplifier 240. Referring again to FIG. 10B, theuncompensated charge waveforms generated by a capacitive sensing node 83(FIG. 3) may be in range of 18 pC_(p-p) (max touch) to 20 pC_(p-p) (notouch). If these signals were provided directly to the inverting inputof the amplifier 240, and if the feedback capacitor (C_(m)) is equal to10 pF, for example, the output of the amplifier will be a voltagewaveform in the range of 1.8 V_(p-p) (max touch) to 2.0 V_(p-p) (notouch). Thus, the dynamic range utilized to sense the difference betweena no touch condition and max touch condition would be only 0.2 V_(p-p),which represents a poor utilization of the dynamic range of theamplifier 240.

FIG. 12 illustrates a compensated charge square waveform 600 when theoutput charge square waveform 404 (FIG. 10B) having an amplitude of 20pC_(p-p), which represents a “no touch” state, is compensated with acompensation waveform 506 (FIG. 11) having an amplitude of 19 pC_(p-p)at the same frequency but 180 degrees out of phase with the outputwaveform 404. The resulting compensated waveform 600 is a chargewaveform having an amplitude of 1 pC_(p-p) with a phase that is the sameas the original output waveform 404. FIG. 12 further illustrates acompensated charge square waveform 602 when the output charge squarewaveform 406 having an amplitude of 18 pC_(p-p), which represents a “maxtouch” state, is compensated with the compensation waveform 506. Theresulting compensated waveform 602 is a charge waveform having anamplitude of 1 pC_(p-p) with a phase that is the same as thecompensation waveform 506 and opposite the phase of the waveform 600.

Thus, with front-end charge compensation, the charge waveform providedto the inverting input of the amplifier 240 swings from +0.5 pC to −0.5pC and its phase shifts 180 degrees as the output levels transition froma “no touch” state to a “max touch” state. If the feedback capacitor(C_(FB)) 242 is equal to 1 pF, for example, the output of the amplifier240 will mirror the inverting input and will swing from +0.5 V to −0.5 Vfrom a “no touch” state to a “max touch” state. Thus, in this example,the utilizing of the dynamic range of the amplifier 240 is increasedfrom 0.2 V_(p-p) to 1.0 V_(p-p), which is a significant improvement.Additionally, the phase of the amplifier output will shift by 180degrees at approximately a midpoint (e.g., a “medium touch” state)during the transition from a “no touch” state to a “max touch” state.This phase shift can be utilized to provide additional informationconcerning the level of pressure being exerted by a touch or a type oftouch. As mentioned above, as a finger is pressed more firmly onto atouch surface, it tends to flatten and increase in surface area, therebystealing more charge from the sensing node and reducing C_(SIG). Thus,the compensated waveform 600 will decrease in peak-to-peak amplitudefrom a “no touch” state to a “medium touch” state, at which point thecompensated waveform 600 is ideally a flat line having an amplitude of 0V_(p-p). As a finger is pressed harder onto the touch surface, thecompensated waveform will transition from a “medium touch” state to a“max touch” state and shift 180 degrees in phase. Additionally, itsamplitude will gradually increase as the finger is pressed down harderuntil the compensated waveform reaches the “max touch” state waveform602, as shown in FIG. 12.

FIG. 13A illustrates an exemplary compensation signal generator 504(FIG. 11) that includes a digital-to-analog voltage converter (VDAC) 610and compensation capacitor (C_(COMP)) 612, in accordance with oneembodiment of the invention. The VDAC 610 receives digital signals froma look-up table (e.g., look-up table 510 in FIG. 11) and generates acorresponding analog voltage signal (e.g., a square wave) having adesired amplitude, frequency and phase. This analog voltage signal isthen provided to C_(COMP) 612, which converts the voltage waveform intoa charge waveform 614 for compensating a charge waveform (e.g., 404 or406) outputted by a touch surface device, for example. This exemplarycompensation circuit provides the benefit of increasing the dynamicrange of the analog sensing circuit, which includes the amplifier 240and feedback capacitor (C_(FB)) 242 discussed above. However, onepotential drawback to this compensation circuit is the necessity of thecompensation capacitor (C_(COMP)) 612. This capacitor will typically beapproximately the same size or on the same order of magnitude in size asC_(F). Therefore, the cost and chip “real estate” savings achieved bythe reduction in size of C_(FB) 242 is offset by the need for C_(COMP)612.

FIG. 13B illustrates another exemplary compensation signal generator 504comprising a digital-to-analog current converter (IDAC) 616, inaccordance with another embodiment of the invention. In this embodiment,IDAC 616 generates a periodic current waveform 618 having a desiredamplitude, frequency and phase based on digital codes received from thelook-up table 510 (FIG. 11). Since current represents a change in chargeover time (or, in other words, charge is the integral of current overtime), the current waveform 618 resembles a square wave when viewed inthe charge domain. Thus, the current waveform 618 can be provideddirectly to the input of the summing circuit 502 (FIG. 11) to compensatethe output charge waveform 404 or 406 from the touch surface device.However, because the current waveform 618 is characterized by aplurality of periodically alternating current spikes, it presentspotentially difficult timing issues when trying to achieve an accurate180 degree phase shift between the current waveform 618 and the outputcharge waveform 404 or 406 from the touch surface device. Thus, the IDAC616 provides both the benefits of increasing the dynamic range of theanalog sensing circuit 500 (FIG. 11) and decreasing IC chip cost and“real estate” requirements due to C (since C_(COMP) is not required).However, the IDAC 616 is less tolerant of phase mismatches and can addto the complexity of the timing logic requirements of the sensingcircuit 500. In one embodiment, in order to minimize phase mismatchesand the effects of any phase mismatches between a compensation signaland an output signal, the invention utilizes the methods and circuitsdescribed in a co-pending and commonly-owned patent application entitled“Minimizing Mismatch During Phase Compensation” (U.S. application Ser.No. 11/650,038), the entirety of which is incorporated by referenceherein.

FIG. 14 illustrates a touch surface system 700 that includes ahigh-voltage level shifter and decoder unit 702 and touch surface panel704, in accordance with one embodiment of the invention. In thisembodiment, the touch surface panel 704 utilizes two touch-insensitiveportions (e.g., top and bottom rows) of the touch surface panel 704 toprovide substantially fixed mutual capacitance values C_(COMP1) andC_(COMP2), respectively. When a 180-degree phase-shifted drive signal isapplied to the top and bottom rows, C_(COMP1) and C_(COMP2) generate twocompensation signals that, when summed, compensate the output signalsprovided by the capacitive nodes of a selected touch-sensitive row, asdescribed above. The level shifter/decoder unit 702 performs functionssimilar to the level shifter 310 and decoder 311 described above withrespect to FIG. 8. In this embodiment, however, the functionality ofthese devices is integrated into a single IC chip 702. The levelshifter/decoder unit 702 receives an input waveform 706 from ASIC 305(FIG. 8), which may be a series of twelve square wave pulses having apeak-to-peak amplitude of 2 V_(p-p), for example. The levelshifter/decoder unit 702 then amplifies this signal to 20 V_(p-p), forexample, and provides this drive signal to a selected row (1−n) based ona timing signal or MUX control signal 708 received from a microprocessor307 (FIG. 8) or the ASIC 305.

The level shifter/decoder unit 702 further includes a plurality ofselection switches 710, which close when a corresponding row has beenselected to be driven by the amplified drive signal. In one embodiment,the level shifter/decoder unit 702 has an output driver 712corresponding to each row of the panel 704. In alternative embodimentsmultiple rows may be connected to the output of one or more drivers 712via a multiplexing/demultiplexing circuit arrangement. The levelshifter/decoder unit 702 further includes an inverting gate 714 whichinverts the incoming drive signal 706 to produce a 180-degreephase-shifted compensation signal that is provided to a top compensationrow (C_(COMP1)) and a bottom compensation row (C_(COMP2)) of the touchsurface panel 704.

The touch surface panel 704 includes a plurality of capacitive sensingnodes, C_(SIG(n, m)), arranged in an (n×m) matrix, where n representsthe number of touch sensitive row electrodes and m represents the numberof column electrodes, which through mutual capacitance, provide outputsignals indicative of touch or no-touch conditions on the panel 704. Thepanel 704 further includes a top row or strip that is touch-insensitiveand provides a substantially fixed capacitance of C_(COMP1). A bottomstrip of the panel is also touch-insensitive and provides asubstantially fixed capacitance of C_(COMP2). As discussed above, thedrive signal applied to the top and bottom touch-insensitive rows is 180degrees out of phase with the drive signal applied to a selectedtouch-sensitive row. Each column electrode is always connected to thetouch-insensitive rows and selectively connected to a touch-sensitiverow (1−n) one at a time. Thus, the compensated capacitance seen at theoutput of each column electrode is effectivelyC_(SIG)−(C_(COMP1)+C_(COMP2)).

In one embodiment, the top and bottom strips of the panel 704 aredesigned so that the values of C_(COMP1) and C_(COMP2) satisfy thefollowing equation: C_(COMP1) C_(COMP2)=(2C_(SIG)−ΔC_(SIG))/2.

In the above equation, ΔC_(SIG) represents the change in mutualcapacitance due to a max touch condition, as discussed above. With thisdesign, the effective compensation signal provided by C_(COMP1) andC_(COMP2) has an amplitude that is equal to the average of the amplitudeof the capacitive sensing node outputs when the node is experiencing a“no touch” state (C_(SIG)) and a “max touch” state (C_(SIG)−ΔC_(SIG)).Some of the advantages of designing the amplitude of the compensationsignal to be equal to the average of the output values corresponding toa “no touch” state and a “max touch” state are discussed above withrespect to FIG. 12.

FIG. 15 illustrates a perspective view of a top portion of the touchsurface panel 704, in accordance with one embodiment of the invention.The panel 704 includes a plurality of row electrodes 706 separated by adielectric layer (not shown) from a plurality of column electrodes 708.In this embodiment, the column electrodes 708 are formed on top of therow electrodes 706 and are substantially orthogonal to the rowelectrodes. However, other configurations and arrangements would bereadily apparent to those of skill in the art. As discussed above, acapacitive sensing node or “pixel” is formed at the intersection of eachrow electrode 706 and each column electrode 708. A mutual capacitance isformed at each node when a drive signal is applied to a correspondingrow electrode (i.e., drive electrode). In this embodiment, the rowelectrodes 706 are configured as the drive electrodes and the columnelectrodes 708 are configured as sense electrodes. In alternativeembodiments, the row electrodes 706 may be configured as the senseelectrodes and the column electrodes 708 may be configured as the driveelectrodes which are driven by a drive input signal applied to the panel704.

The top row 710, however, is touch-insensitive due to the configurationand arrangement of the top portions 712 of the column electrodes 708above the top row 710. As shown in FIG. 15, the top portions 712 of thecolumn electrodes 708 are expanded to substantially cover the entiretyof the top row 710, thereby shielding the mutual capacitance (C_(COMP1))formed between the top row 710 and respective top portions 712 of thecolumn electrodes 708 from any shunting effects that would otherwise becaused by a finger or other object touching the top portion of the panel704. Although the top portions 712 are expanded, they do not makeelectrical contact with adjacent top portions 712. A bottomtouch-insensitive row (not shown) is formed in a similar fashion.

It is understood in alternative embodiments there may only be onetouch-insensitive row, or any number of desired touch-insensitive rowsor columns. For example, the insensitive portions of the panel 704 maybe configured along one or both side edges of the panel 704 instead ofthe top and bottom edges. In such a configuration, the row electrodes706 can be formed on top of the column electrodes 708 with the ends ofthe row electrodes 706 expanded in a similar manner as the expandedportions 712 of the column electrodes 708.

FIGS. 16A and 16B illustrate the effects of a finger touching atouch-sensitive and a touch-insensitive portion of the panel 704,respectively. Referring to FIG. 16A, when a finger 800 or other objecttouches or comes in sufficient proximity to (collectively referred to asa “touch” herein) a sensing node formed at an intersection of a columnelectrode 708 and a row electrode 706, the finger or object blocks or“steals” some of the electric field lines 802 between the column 708 androw 706. These electric field lines are shunted to ground through thecapacitance inherent in the finger, and as a result, the steady statesignal capacitance C_(SIG) is reduced by ΔC_(SIG). Therefore, the signalcapacitance at the node becomes C_(SIG)−ΔC_(SIG), where C_(SIG)represents the static (no touch) component and ΔC_(SIG) represents thedynamic (touch) component.

FIG. 16B illustrates the shielding effects of the expanded portions 712of the column electrodes 708 when a finger touches the corresponding topor bottom portion of the panel 704. Since the expanded portions 712 ofthe column electrodes substantially cover the entirety of the underlyingrow electrode 710, the finger 800 is shielded from the underlyingelectrodes and does not affect the electric field lines 802 between theelectrodes. Thus, there is no shunting effect or sensitivity to a touchevent at this portion of the panel 704. As discussed above, thetouch-insensitive top and bottom rows or portions of the panel 704 maybe designed to provide desired values of C_(COMP1) and C_(COMP2),respectively. Those of skill in the art would readily know how toachieve such desired mutual capacitance values in order to generate adesired compensation signal as described herein.

One advantage of generating the compensation signal through the panel704, as described above, is that the compensation signal willsubstantially “track” any variations in the mutual capacitance (C_(SIG))values present at the touch-sensitive portions of the panel 704 that maybe due to, for example, variations in operating parameters (e.g.,temperature) and/or processing parameters (e.g., dielectric thickness).Thus, the compensation signal will mimic any variations in the outputsignals from the touch-sensitive portions of the panel 704. Onedisadvantage, however, is that the effective surface area of the panel704 for receiving touch inputs is slightly reduced.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not of limitation. For example, although thedisclosure is primarily directed at devices that utilize capacitivesensing, some or all of the features described herein may be applied toother sensing methodologies. Additionally, although embodiments of thisinvention are primarily described herein for use with touch sensorpanels, proximity sensor panels, which sense “hover” events orconditions, may also be used to generate modulated output signals fordetection by the analog channels. Proximity sensor panels are describedin Applicants' co-pending U.S. application Ser. No. 11/649,998 entitled“Proximity and Multi-Touch Sensor Detection and Demodulation,” filedconcurrently herewith as Attorney Docket No. 106842001100, the contentsof which are incorporated herein by reference in its entirety. As usedherein, “touch” events or conditions should be construed to encompass“hover” events and conditions and “touch surface panels” should beconstrued to encompass “proximity sensor panel.” Likewise, the variousdiagrams may depict an example architectural or other configuration forthe invention, which is done to aid in understanding the features andfunctionality that can be included in the invention. The invention isnot restricted to the illustrated example architectures orconfigurations, but can be implemented using a variety of alternativearchitectures and configurations. Additionally, although the inventionis described above in terms of various exemplary embodiments andimplementations, it should be understood that the various features andfunctionality described in one or more of the individual embodiments arenot limited in their applicability to the particular embodiment withwhich they are described, but instead can be applied, alone or in somecombination, to one or more of the other embodiments of the invention,whether or not such embodiments are described and whether or not suchfeatures are presented as being a part of a described embodiment. Thusthe breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” shouldbe read as mean “including, without limitation” or the like; the term“example” is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; and adjectivessuch as “conventional,” “traditional,” “normal,” “standard,” “known” andterms of similar meaning should not be construed as limiting the itemdescribed to a given time period or to an item available as of a giventime, but instead should be read to encompass conventional, traditional,normal, or standard technologies that may be available or known now orat any time in the future. Likewise, a group of items linked with theconjunction “and” should not be read as requiring that each and everyone of those items be present in the grouping, but rather should be readas “and/or” unless expressly stated otherwise. Similarly, a group ofitems linked with the conjunction “or” should not be read as requiringmutual exclusivity among that group, but rather should also be read as“and/or” unless expressly stated otherwise. Furthermore, although items,elements or components of the invention may be described or claimed inthe singular, the plural is contemplated to be within the scope thereofunless limitation to the singular is explicitly stated. The presence ofbroadening words and phrases such as “one or more,” “at least,” “but notlimited to” or other like phrases in some instances shall not be read tomean that the narrower case is intended or required in instances wheresuch broadening phrases may be absent.

What is claimed is:
 1. A front-end signal compensation apparatus,comprising: a compensation circuit configured for coupling to a sensenode and receiving a sense signal indicative of a detected touch, thesense signal having a first amplitude during a no-touch condition and asecond amplitude during a touch condition, the first amplitude beinggreater than the second amplitude; and a signal generator coupled to thecompensation circuit and configured for generating a compensation signal180 degrees out of phase with respect to the sense signal, thecompensation signal having a third amplitude between the first amplitudeand the second amplitude; wherein the compensation circuit is configuredfor summing the sense signal with the compensation signal to produce acompensated sense signal having the same phase as the sense signalduring the no-touch condition and 180 degrees out of phase with respectto the sense signal during the touch condition.
 2. The front-end signalcompensation apparatus of claim 1, further comprising: an amplifierhaving an inverting input coupled to the compensation circuit and anon-inverting input coupled to a known reference voltage; and a feedbackcapacitor coupled between an output of the amplifier and the invertinginput.
 3. The front-end signal compensation apparatus of claim 1,wherein the signal generator comprises: a digital-to-analog converter(DAC) for generating the compensation signal, the DAC coupled to thecompensation circuit; and a look-up table, coupled to the DAC, forstoring and providing a digital code to the DAC, wherein thecompensation signal is generated based on the digital code.
 4. Thefront-end signal compensation apparatus of claim 3, wherein the DACcomprises a digital-to-analog voltage converter (VDAC) and a capacitorcoupled to the VDAC.
 5. The front-end signal compensation apparatus ofclaim 3, wherein the DAC comprises a digital-to-analog current converter(IDAC).
 6. The front-end signal compensation apparatus of claim 1,wherein the third amplitude of the compensation signal is selected to bean average of the first and second amplitudes of the sense signal. 7.The front-end signal compensation apparatus of claim 2, furthercomprising a mixer coupled to the amplifier and the signal generator fordemodulating the output of the amplifier.
 8. The front-end signalcompensation apparatus of claim 7, further comprising ananalog-to-digital converter (ADC) coupled to the mixer for generating ademodulated compensated sense signal.
 9. A touch sensing systemincorporating the front-end signal compensation apparatus of claim 1,the touch sensing system comprising a touch sensor configured forgenerating the sense signal.
 10. A method for front-end signalcompensation, comprising: receiving a sense signal indicative of adetected touch, the sense signal having a first amplitude during ano-touch condition and a second amplitude during a touch condition, thefirst amplitude being greater than the second amplitude; generating acompensation signal 180 degrees out of phase with respect to the sensesignal, the compensation signal having a third amplitude between thefirst amplitude and the second amplitude; and summing the sense signalwith the compensation signal to produce a compensated sense signalhaving the same phase as the sense signal during the no-touch conditionand 180 degrees out of phase with respect to the sense signal during thetouch condition.
 11. The method of claim 10, further comprisingamplifying the compensated sense signal with a virtual-ground amplifier.12. The method of claim 10, further comprising generating thecompensation signal by: accessing a look-up table to generate a digitalcode; and providing the digital code to a digital-to-analog converter(DAC) to generate the compensation signal.
 13. The method of claim 10,further comprising selecting the third amplitude of the compensationsignal to be an average of the first and second amplitudes of the sensesignal.
 14. The method of claim 11, further comprising demodulating theamplified and compensated sense signal.
 15. The method of claim 14,further comprising converting the demodulated, amplified and compensatedsense signal from an analog signal to a digital signal.
 16. A front-endsignal compensation apparatus, comprising: means for receiving a sensesignal indicative of a detected touch, the sense signal having a firstamplitude during a no-touch condition and a second amplitude during atouch condition, the first amplitude being greater than the secondamplitude; means for generating a compensation signal 180 degrees out ofphase with respect to the sense signal, the compensation signal having athird amplitude between the first amplitude and the second amplitude;and means for summing the sense signal with the compensation signal toproduce a compensated sense signal having the same phase as the sensesignal during the no-touch condition and 180 degrees out of phase withrespect to the sense signal during the touch condition.
 17. Thefront-end signal compensation apparatus of claim 16, further comprisingmeans for amplifying the compensated sense signal.
 18. The front-endsignal compensation apparatus of claim 16, the means for generating thecompensation signal comprising: table look-up means for generating adigital code; and means for converting the digital code to an analogsignal to generate the compensation signal.
 19. The front-end signalcompensation apparatus of claim 16, further comprising means forselecting the third amplitude of the compensation signal to be anaverage of the first and second amplitudes of the sense signal.
 20. Thefront-end signal compensation apparatus of claim 17, further comprisingmeans for demodulating the amplified and compensated sense signal.